By the use of Indium-111-oxine labeled platelets, together with multiple isotopic markers, the proposed experiment will attempt to determine the fate of platelets in terms of true tissue biodistribution in normal animals. In doing so, we expect to establish whether, in normal control animals, with undamaged vascular endotheliums, platelets are simply destroyed by the reticuloendothelial system, or whether a more complex situation obtains, involving an adherence of some platelets to the "normal" vascular endothelium (in addition to reticuloendothelial destruction of the vast majority of aging platelets). After this characterization is completed, the effect of stress, both acute and repeated, on the fate and biodistribution of Indium-111 labeled platelets will be studied. In particular, stress experiments will attempt to determine if the platelets are causing myocardial damage or rather resulting from it. In addition to the platelet study, two behavioral experiments will be undertaken. First, the effect of early coping will be studied, in which groups of animals are trained to either avoidance or escape behavior in the face of stress, and then subjected to unavoidable stress. Second, the effect of chronic exercise training on stress-induced myocardial damage will be studied.